An engine firing order is the planned sequence in which cylinders produce power. In a four-stroke engine, each cylinder must complete intake, compression, power, and exhaust strokes in the correct timing. If cylinders fired randomly, the engine would shake, lose power, and stress the crankshaft.
Learning firing order helps students understand how smooth rotation, balance, and reliable power are produced in a vehicle engine.
In an inline 4-cylinder engine, a common firing order is 1-3-4-2, with each cylinder firing once every two crankshaft revolutions. Since two crankshaft revolutions equal 720 degrees, the firing events are spaced 180 degrees apart. The crankshaft, camshaft, spark plugs, valves, pistons, and connecting rods all work together so each combustion event happens at the right moment.
This sequence spreads force along the crankshaft and keeps the engine running smoothly through the full cycle.
Key Facts
- Firing order is the sequence in which engine cylinders produce power strokes.
- A four-stroke engine cycle takes 720 degrees of crankshaft rotation.
- In an inline 4-cylinder four-stroke engine, firing events are usually spaced 720 degrees / 4 = 180 degrees apart.
- A common inline 4-cylinder firing order is 1-3-4-2.
- Spark timing must occur near the end of the compression stroke, just before the power stroke.
- Camshaft speed = crankshaft speed / 2 in a four-stroke engine.
Vocabulary
- Firing order
- The firing order is the planned sequence in which the cylinders ignite their air-fuel mixture.
- Crankshaft
- The crankshaft is the rotating shaft that converts piston motion into rotational motion for the vehicle.
- Camshaft
- The camshaft opens and closes the intake and exhaust valves at the correct times.
- Power stroke
- The power stroke is the part of the engine cycle when combustion pushes the piston downward to produce useful work.
- Top dead center
- Top dead center is the highest position of a piston in its cylinder.
Common Mistakes to Avoid
- Assuming cylinders fire in physical order 1-2-3-4 is wrong because the firing order is chosen to balance forces and reduce vibration.
- Forgetting that a four-stroke cycle takes 720 degrees is wrong because each cylinder fires only once every two crankshaft rotations.
- Confusing piston position with firing moment is wrong because a piston near top dead center may be finishing compression or finishing exhaust.
- Ignoring camshaft timing is wrong because spark, intake valve opening, and exhaust valve opening must be synchronized for the engine to run correctly.
Practice Questions
- 1 An inline 4-cylinder four-stroke engine uses firing order 1-3-4-2. If cylinder 1 fires at 0 degrees, which cylinders fire at 180 degrees, 360 degrees, and 540 degrees?
- 2 A 4-cylinder engine runs at 2400 rpm. How many firing events occur per minute if each cylinder fires once every two crankshaft revolutions?
- 3 Explain why an engine designer would choose a firing order such as 1-3-4-2 instead of making the cylinders fire in the physical order 1-2-3-4.